In antenna systems that comprise a plurality of antennas, the signals received from the plurality of antennas must be combined to form a coherent beam. Alternatively, a coherent beam must be divided into separate signals for transmission from a plurality of antennas. In antenna arrays where the antenna elements are spatially close together, only phase delays are required to accomplish beamforming. When the antenna elements are not spatially close together, time delays are also required to accomplish beamforming.
In some installations, a plurality of antenna elements is utilized wherein individual antenna elements are “diverse”, that is, are different in location, orientation, size and other aspects. Such an installation requires precise control over time delay, phase delay and amplitude to achieve coherent beamforming. Phase continuous true time delay circuits are known that accomplish the coherent beamforming from diverse antenna arrays to provide higher communication data rates, reduced power requirements and wider coverage areas.
Commonly owned U.S. Pat. No. 7,009,560, incorporated by reference herein in its entirety, discloses an adaptive variable true time delay beamforming system. In one disclosed embodiment, the beamforming system is utilized to characterize a received signal and to calibrate an antenna system that includes a variable true time delay system. However, known true time delay systems utilize RF cables or optical fibers having different lengths to impart a time delay, making physical manipulation of these cables or optical fibers required for any calibration. Moreover, known systems require periodic maintenance after system deployment, and further require periodic testing, calibration and performance verification, especially if the RF cables or optical fibers are interchanged in the field.
It is desirable to develop a method and system to automatically calibrate a variable true time delay beamforming system applicable to both transmission and reception of RF signals that greatly reduces or eliminates the need for periodic tests, calibration and performance verifications in the field.